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Topic: Help ID'ing NJ rock? (Read 3007 times)

Hello folks, found this odd looking rock in a creek in NJ. It's about the size of a large grapefruit and maybe 6-7 lbs. A magnet wouldn't stick to it at all, nor could I scratch it with anything handy (tried hard stainless steel with no results). I thought maybe it was some sort of molten metal due to the many small stones embedded within. Any suggestions? Thanks in advance for any assistance!

I certainly make no claim to be a "proper geologist", just a somewhat out of date amateur.

The only puddingstone with which I have had personal contact is Hertfordshire puddingstone (UK). That is composed of flint pebbles in a silica matrix. That would accord well with the hardness described in the OP. Also there are signs that the rock has fractured through, as well as around the clasts (pebbles), which suggests that the matrix is as hard as the clasts. If I were a betting man I'd put money on RD's "guess".

Thanks to everyone for your replies! This is the other side of this rock, as you can see there are larger stones embedded here and there, and there are a few indentations where stones appeared to have separated leaving smooth rounded crater-like indentations.

Amygdales form when vesicles in the basalt are subsequently infilled with secondary minerals. This requires that the basalt be porous enough to be penetrated by these minerals. Of course, in nature, things are rarely as simple as we might like them to be. I have collected specimens in Cornwall in which the basalt seemed so impervious that it was difficult to imaging how anything could penetrate it. However, closer investigation showed that the basalt had been subjected to a degree of metamorphism which was easy to overlook.

Have you checked the hardness of the clasts in your specimen? If they are soft, eg calcite or a zeolite, the chances are they are amygdales. If they are quartz, you are back to square one. Just from the appearance of your specimen I would still go for puddingstone, but would have to agree that the matrix is very dark.

My US geography is not good, so I donít know if the locality would throw any light on the problem. It might be worth your while looking up some local reference works, if you have not done that already.

Looking at the pic of "the other side" of your specimen, a couple of points are worth mentioning.

The large clast at the top looks like a weathered quartz pebble, whereas the large one on the right looks quite different; a wild guess might be that this is quartzite; if so, that would support the conglomerate origine.

It seems that the clasts range in size from the large ones already mentioned, down to sand grain size. It would be worth checking local geological literature for a depositional history that might include unsorted material of this kind that could later have been cemented by silica.

You would still have to account for how the final rock became matrix-supported, but Bass is probably the person to deal with that.

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